title: Two-level noncontiguous versus three-level anterior cervical discectomy and fusion: a biomechanical comparison.
abstract: METHODS
  Biomechanical study.
  
  
  OBJECTIVE
  To determine biomechanical forces exerted on intermediate and adjacent segments after two- or three-level fusion for treatment of noncontiguous levels.
  
  
  BACKGROUND
  Increased motion adjacent to fused spinal segments is postulated to be a driving force in adjacent segment degeneration. Occasionally, a patient requires treatment of noncontiguous levels on either side of a normal level. The biomechanical forces exerted on the intermediate and adjacent levels are unknown.
  
  
  METHODS
  Seven intact human cadaveric cervical spines (C3-T1) were mounted in a custom seven-axis spine simulator equipped with a follower load apparatus and OptoTRAK three-dimensional tracking system. Each intact specimen underwent five cycles each of flexion/extension, lateral bending, and axial rotation under a ± 1.5 Nm moment and a 100-Nm axial follower load. Applied torque and motion data in each axis of motion and level were recorded. Testing was repeated under the same parameters after C4-C5 and C6-C7 diskectomies were performed and fused with rigid cervical plates and interbody spacers and again after a three-level fusion from C4 to C7.
  
  
  RESULTS
  Range of motion was modestly increased (35%) in the intermediate and adjacent levels in the skip fusion construct. A significant or nearly significant difference was reached in seven of nine moments. With the three-level fusion construct, motion at the infra- and supra-adjacent levels was significantly or nearly significantly increased in all applied moments over the intact and the two-level noncontiguous construct. The magnitude of this change was substantial (72%).
  
  
  CONCLUSIONS
  Infra- and supra-adjacent levels experienced a marked increase in strain in all moments with a three-level fusion, whereas the intermediate, supra-, and infra-adjacent segments of a two-level fusion experienced modest strain moments relative to intact. It would be appropriate to consider noncontiguous fusions instead of a three-level fusion when confronted with nonadjacent disease.
authors.affiliation: Department of Neurosurgery, University of Utah School of Medicine, Salt Lake City, UT, USA.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Finn
authors.forename: Mical M
authors.initials: MM
authors.lastname: Samuelson
authors.forename: Frank
authors.initials: F
authors.lastname: Bishop
authors.forename: Kent N
authors.initials: KN
authors.lastname: Bachus
authors.forename: Darrel S
authors.initials: DS
authors.lastname: Brodke
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D002102:Cadaver; D002574:Cervical Vertebrae / Q000503:physiopathology / Q000601:surgery*; D017586:Diskectomy / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D007403:Intervertebral Disc / Q000503:physiopathology / Q000601:surgery*; D008875:Middle Aged; D016059:Range of Motion, Articular; D012399:Rotation; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*